The present invention relates to invert emulsions for the well-drilling industry and to a method for their preparation. More particularly, the present invention relates to compositions comprising an emulsion of a brine in a liquid hydrocarbon which are useful in well-drilling operations, such as in gas- and/or oil-well drilling, as a drilling fluid, a completion fluid, a workover fluid, a casing pack fluid, a fracturing fluid, a packer fluid and a spacer fluid.
Invert emulsions, in the form of drilling muds, completion fluids and packer fluids are well known in the well-drilling art. Invert emulsions typically consist of a fluid phase comprising from 15 to 45 percent by volume of a CaCl.sub.2 brine, and 55 to 85 percent by volume of diesel oil and a solid phase comprising agents for pressure control, filtration control, viscosity control, gelling, etc. An invert emulsion drilling mud generally contains from 5 to 30 percent by volume of the brine.
While conventional invert emulsions are used in drilling deep wells, where high temperature and pressure are commonly encountered, they are not completely satisfactory for such a use. For example, invert emulsions are sometimes not sufficiently stable at the high temperatures encountered in deep wells. Furthermore, the use of excessive amounts of a weighting agent in an invert emulsion to achieve pressure control in deep wells is often undesirable because performance and properties of the emulsion can be adversly affected. Heavier brines, such as CaBr.sub.2 brines and/or ZnBr.sub.2 brines, have been used to increase the density of an invert emulsion, thereby achieving greater pressure control; however, sometimes the thermal stability of these heavier emulsions is marginal or non-existent. Further improvements in pressure control and in temperature stability of the invert emulsion are needed.